Let us compare the OSI layer with an example, suppose a Manager of an office, wants to invite his friend to a dinner, who is working in a different office sharing the same Rank:

User

Boss

Decides to send a parcel to his friend

Application

Application used

He posts a letter, and informs his PA about it.

Presentation

Personal assistant (PA)

PA says, “Ok Sir! I will forward this parcel, if needed I will do the job of translation and encryption and passes the message to Legal Adviser!"

Session

Legal Advisor

On getting the parcel, he checks it, and decides whether it is wise to send it and checks if their office is in good terms with the different office; and then he passes the message Mail Manager

Transport

Mail Manager

He gets the parcel, and then he thinks that “Oh My God! Such a big parcel, let me break the parcel and make segments(chunks) of it ”, he further numbers the chunks so that it can be combined to make the parcel by the receiving party. Next he contacts the Mail manager of the receiving office, and says, “hey I will be sending your office some segments, please let me know when you receive the segments” and then passes the segments to Mail Room.

Network

Mail Room

When the Mail Room receives the segments, people working there, adds From and To address to each segments, and now the segments are known as packets. And now these packets are handed to Mail Service Department.(Mail Room is more concerned about the location address of the receiving party, but not the exact address, which is the house number)

Data Link

Mail Service

When the Mail Service Department, receives the packets it adds the physical address of the office, which is the house number, and then makes a check of the packets for any mismatch or loss by the above layers. Then places the result on the packets, and now what we have is a Frame. Again the Frame has to have some kind of medium so that it can reach the receiving party. It decides the medium and then hands over to the decided medium.

Physical

Medium

The Frames are converted to BITS, before it was handed to the medium. Now medium, which can be a Van, Train, Cargo, Airplane, etc..takes it, and starts for the receiving party.

Now, there must be a Driver driving the medium, let’s say a cargo van, the medium doesn’t know without the driver where to go. Here Driver acts as a Router, and he makes decision based to network layer address (Layer 3), and drives to the exact location (Routes to the correct network).

But again the driver doesn’t care about the exact house number, he delivers it to door of the location (network) and his job is done. Now they are picked up by the guard, and takes it to the Data Link (Mail service Department), what he receives is a Frame, they verifies and replies “Oh yes! These are for us! Thank you!” They then removes the physical address header from the packets and passes it to the Mail Room, they verifies and replies “Ohh! Yes, these packets are meant for us. Thank You”, they removes the Logical Address (To and From Address) and now we have segments, which are handed to Mail Manager, he combines the segments based on the numbering and labeling done by the sending Mail Manager, and Acknowledges the Sending Mail Manager “Hey, I just received all the segments. Thank You!” now after combining the segments what we have is the parcel and it is handed to Legal Advisor (Session Layer), he checks and decides “Umm!! So it seems this parcel is for our boss, well I will process the parcel as we are in a good relation with the sender, it will not contain any harmful materials”, he then hands it to the Personal Assistant, and says “This message is for our Boss, please see that before you transfer you do your job of Translation or Decryption is Required”, PA then checks if there is anything to Translate or Decrypt and then hands it to Application Layer, now the Application layer reads the letter for the Boss..

”Hi Mr. X,

It has been a long time that we have met each other, let us meet for a dinner party..Blah blah blah!!”

So wasn’t that easy?? Huh!! That really was..Believe me it will really help you to correlate when I actually describe the functions in terms of Data Transfer in a network following the OSI Layer.

Thank You!

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14 comments:

Thank you Subir, the OSI model is quite a difficult area for me and I this is helping a lot. What I am going to do is print of your table and then add pictures to it (as this will help me more!) :-) All the best. Jon

Thanks a bunch. I was actually thinking of how the layers work together and half way through I thought "Wait a minute, I wonder if someone already's done it". Thanks for doing most of the work for me ;)No I'm going to translate it to when IE is accessing a webpage and my review-material is complete.Keep up the good work

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